1. Field of The Invention
The present invention relates to a novel catalyst structure and the process for the preparation of nitric acid employing the structure.
2. Relevant Art
Nitric acid is made by oxidizing ammonia over a wire mesh gauze catalyst produced by weaving wires of precious metals (e.g. platinum, Ostwald process) and their alloys. The process operates at temperatures from 800.degree. to 1100.degree. C. The hot nitric oxide formed is quenched and air is added so the nitric oxide oxidizes to NO.sub.2 (nitrogen (IV) oxides) which reacts with water to form nitric acid. The wire mesh gauze is alloyed to increase high temperature strength and to reduce costs. Typically, platinum is used as the primary catalyst and is alloyed with ruthenium to improve high temperature strength and palladium to reduce costs. The prior art in the nitric acid process, determined that a mass of catalytic material, e.g., platinum was necessary because of the continual erosion of the platinum by volatilization.
Ammonia oxidation catalysts for nitric acid production are based on precious metal fine wires woven together on a loom into the form of a gauze. The typical gauze contains 1.78 troy ounces of platinum or precious metal alloy per square foot. For some plants as many as 27-30 of these gauzes each 60 to 120 inches in diameter may be stacked. These gauzes will last for six weeks to a year depending on the severity of use (high pressure versus low pressure) and can fail for as variety of reasons including burn through (caused by channeling or difficult start up) and loss of platinum metal due to volatilization.
The conventional catalysts are fragile and lose platinum at high temperature via PtO.sub.2 volatilization. Other gauzes (getter gauzes) are used downstream to capture the volatile platinum species through a collection and reduction process.
The gauze catalysts are composed of screens with 3 to 30 stacked screens used. A high pressure oxidation process requires more screens and results in higher losses of platinum metal. This stacked screen configuration is dimensionally unstable in that it expands and sags and allows non-uniform flow and temperature distribution and poor mixing.
Fine wires are desired for improved catalyst performance but result in poor physical integrity and reduced life. The problems with the platinum metal alloy gauzes begin at start-up. It has been concluded that this reaction carried out over precious metal mesh is mass transfer limited. Most gauze reactors are over designed in terms of the number of gauzes. The over design is required to compensate for poor utilization of catalyst due to poor flow distribution. Poor flow distribution due to non uniformity and low pressure drop causes "flickering", a condition in which increased flow through a gauze section can release more heat. These high temperature "hot spots" can lose platinum as the oxide at the higher temperature. The non uniformity and low pressure drop of gauzes gives rise to additional loss of efficiency if the gases are not thoroughly mixed, as little additional mixing is achieved by the gauze catalyst structure.
One attempt to solve the platinum loss is shown in U.S. Pat. No. 4,863,893. Another approach is described by Robert J. Farrauto and Hgo C. Lee in "Ammonia Oxidation Catalyst with Enhanced Activity", Ind. Eng. Chem. Res. Vol. 29, No. 7, 1990, pages 1125-1129 where high surface area is obtained by depositing platinum from solution on the platinum alloy gauze.
Ceramics and many metals are strong, resistant to corrosion, resistant to relatively high temperatures and would generally be desirable as structural materials in the nitric acid process and may be used in various components. Conventional ceramics were not suitable because of insufficient porosity and an insufficient quantity of platinum to allow for volatilization loss.
In several full size industrial reactors reticulated ceramic foam structures have been successfully tested as a flow distributor, thermal radiation guard and physical support. These guard beds have been shown to improve the life of the gauze and performance and to withstand the harsh chemical environment and high reaction temperature. The foams can be produced in a variety of cell densities with the most common being in the range of 10 to 70 pores per linear inch.
Reticulated ceramics and metals have been employed as filters, scrubbers, packing supports and more recently in automobile catalytic exhaust converters. The reticulated ceramics were initially developed for filtration of molten metals.
Various reticulated ceramic structures are described in the art: U.S. Pat. No. 4,251,239 discloses flutted filter of porous ceramic having increased surface area; U.S. Pat. No. 4,568,595 discloses reticulated ceramic foams with a surface having a ceramic sintered coating closing off the cells; U.S. Pat. No. 3,900,646 discloses ceramic foam with a nickel coating followed by platinum deposited in a vapor process; U.S. Pat. No. 3,957,685 discloses nickel or palladium coated on a negative image ceramic metal/ceramic or metal foam; U.S. Pat. No. 3,998,758 discloses ceramic foam with nickel, cobalt or copper deposited in two layers with the second layer reinforced with aluminum, magnesium or zinc; U.S. Pat. No. 4,863,712 discloses a negative image reticulated foam coated with cobalt, nickel or molybdenum coating; U.S. Pat. No. 4,308,233 discloses a reticulated ceramic foam having an activated alumina coating and a noble metal coating useful as an exhaust gas catalyst; U.S. Pat. No. 4,253,302 discloses a foamed ceramic containing platinum/rhodium catalyst for exhaust gas catalyst; and U.S. Pat. No. 4,088,607 discloses a ceramic foam having an active aluminum oxide layer coated by a noble metal containing composition such as zinc oxide, platinum and palladium.
Catalyst sites for precious metals are maximized by deposition of small crystallites, usually from solution. The use of chlorination to produce precious metal solutions is universal. Many catalysts are produced from these chloride solutions via hydrolysis or precipitation and in some cases by drying and thermal reduction. In all these cases there are residual halides.
Certain gaseous species can adsorb so strongly that they tie up the sites preventing catalysis or in some cases the adsorbed species react with the surface sites to form new chemical compounds which may undergo structural (chemical and physical) change. Examples of these type of poisons are SO.sub.2, Cl.sub.2, H.sub.2 S, I.sub.2, F.sub.2, etc.
Improved catalysts have previously been found which are prepared from chloride free solutions. Examples are nitrates of palladium and rhodium and the hexa hydroxy platinates and platinum sulfito complexes.
It is an advantage of the present invention to provide an improved catalyst for ammonia oxidation with increased performance (yield and selectivity), longer life and with greatly reduced costs by depositing metallic catalysts on ceramic or metallic structures, which are better able to withstand upsets and not result in burn through.
It is a further advantage that the reticulated structure provides high surface area and uniform and controllable porosity. These properties give excellent gas contacting and uniform pressure drop which can be critical for good catalyst performance for ammonia oxidation. Furthermore, the reticulated structure provides a structure which presents a surface of catalytic material but unlike the wire gauze catalyst the inner core is inert material and not costly and inaccessible precious metal.
It is a feature of the present invention that catalysts in which the precious metals are produced from metallo organic compounds dissolved in organic solvents which have superior catalytic activity (conversion at lower temperature) and stability. These catalysts have superior properties to those produced by known and practiced technology based on inorganic chloride or chloride free aqueous compounds.